1. Field of the Invention
The invention relates to high intensity discharge (HID) lamps. More specifically, the invention relates to a frame assembly for HID lamps in which a discharge vessel, such as an arc tube, is canted at a predetermined angle with respect to the lamp axis. The invention also relates to a rotary locking joint for connecting sections of the frame.
2. Description of the Prior Art
In the majority of HID lamps, the discharge vessel is arranged in the outer envelope aligned with the lamp axis. The outer envelope has a bulbous portion surrounding the arc tube, and an elongate neck portion extending from the bulbous portion. The arc tube is normally supported in the lamp envelope by a support frame fixed to rigid current-supply conductors extending from the lamp stem through the neck portion of the outer envelope.
The burning orientation of the arc tube in HID lamps affects the efficiency of the lamps. It has been found that a vertical burning position of the arc tube is most efficient, followed by a horizontal burning position. However, in certain lamp applications, it is not practicable to orient the outer bulb in a horizontal or vertical position. For example, in portable light towers for illuminating sports playing fields, the design and position of the reflector for illuminating the playing field limits the orientation of the outer bulb to a base-up orientation with the bulb axis between the vertical and horizontal. Thus, such luminaires would support the arc tube at an angle which is less efficient than the preferred vertical or horizontal burning positions. As a result, prior art lamps having an arc tube aligned with the lamp axis do not provide optimum light distribution or efficiency when operated in such luminaires.
A proposed solution has been to provide a lamp in which the arc tube is supported in the outer envelope at a predetermined angle with respect to the lamp axis. For sports lighting applications, the arc tube is canted such that it would be in a horizontal burning position when secured in a luminaire. However, in lamps having an arc tube canted more than only a few degrees from the lamp axis, the configuration of the frame for supporting the arc tube has prevented the insertion of the frame and arc tube through the elongate neck portion of the standard bulged-tube (BT) outer envelope.
U.S. application Ser. No. 07/312,504, filed Feb. 17, 1989, now U.S. Pat. No. 4,936,807 discloses a solution to the assembly problem which involves severing of the neck portion of the outer envelope from the bulbous portion. A stem assembly having rigid conductors extending from the stem press is first sealed to the neck portion. After severing the neck portion near the bulbous portion, a frame holding the arc tube and having rigid conductors bent for holding the arc tube at the preferred angle is welded to the rigid conductors. The arc tube and frame are then maneuvered through the severed opening and the neck portion is resealed to the bulbous portion. However, this assembly method requires additional equipment for the severing and resealing of the neck portion and is labor intensive, making it comparatively inefficient and expensive compared to the known assembly method for lamps having arc tubes aligned with the lamp axis.
Frame assemblies for canted arc tubes which allow the frame assembly with the arc tube to be inserted through the neck portion of the outer envelope and permit pivoting of the arc tube to the canted position are known in the art. U.S Pat. No. 2,904,710 discloses a frame assembly for securing the arc tube transverse to the lamp axis. The frame assembly has a support aligned with the lamp axis and a cradle for holding the arc tube which is pivotable about the support at the center of the cradle. The cradle is rotated into the transverse position and latched by a latching mechanism prior to sealing of the reentrant stem in the neck portion of the envelope. However, an additional wire separate from the frame and enclosed by an insulator tube is required to energize one of the discharge electrodes.
U.S Pat. No. 4,142,122 discloses a frame with two spaced and curved rods on which the ends of the arc tube slide allowing the arc tube to be pivoted to a transverse position by a coil spring. This frame support has the disadvantage that the arc tube is not locked in the transverse position but is held only by the force of the spring biasing the arc tube ends against pivot stops on the curved rods.
Frame assemblies in which the arc tube is pivotable about one end at a hinge connection with a rigid frame conductor are known from U.S. Pat. No. 4,422,004, U.S. Pat. No. 4,410,828, and JP Kokai No. 59-158064. In U.S Pat. No. 4,410,828, the arc tube is latched in the transverse position by a wire portion secured to the arc tube which cooperates with a notched double-wire clasp spring fixed to the rigid frame conductor. In U.S. Pat. No. 4,422,004, a two-piece connector having a bendable knee joint and extending from one of the conductors in the stem to the lead-in of the arc tube opposite the hinge joint secures the arc tube in the transverse position. These two lamps have the disadvantage that the arc tube must be pivoted by a wand inserted into the envelope during lamp assembly. In JP Kokai No. 158064, a wire of shape memorizing alloy is attached to the end of the support on which the arc tube has a slide connection. During assembly, the wire is relaxed and permits passage of the arc tube through the neck portion. At elevated temperatures, the wire becomes coiled and draws the arc tube into the transverse position. This construction has the disadvantage that the arc tube is secured in position only by the memory alloy wire.